Capacitors are common electrical parts which are primarily used for power supply circuits or digital circuit noise filters in various types of electrical and electronic products. Capacitors are largely classified into electrolytic capacitors and non-electrolytic capacitors (ceramic capacitors, film capacitors, etc.).
A wide variety of electrolytic capacitors are currently used, such as aluminum capacitors, wet tantalum electrolytic capacitors, and the like. Because aluminum electrolytic capacitors are able to provide a particularly excellent effect of the invention, the following explanation of the invention will be particularly focused on aluminum electrolytic capacitors, but the invention may be applied generally to all types of electrolytic capacitors without limitation.
A valve metal is used as the electrode material for an electrolytic capacitor and, in an aluminum electrolytic capacitor, aluminum is used as the electrode material. The basic structure of an electrolytic capacitor comprises an element comprising electrodes of prescribed shape having a dielectric oxide film on the surface (etched, if necessary, to increase the surface area and control capacitance) as the anode and cathode, arranged in a mutually opposing manner, with an electrolyte solution-holding separator lying between them. The element of the electrolytic capacitor is sealed into a package. An electrolytic capacitor element may have a wound structure or a laminated structure.
In an electrolytic capacitor, such as that described above, the properties of the electrolyte solution constitute a major factor determining the performance of the electrolytic capacitor. Particularly, with the downsizing of electrolytic capacitors in recent years, anode foils or cathode foils with higher etching factors have been used and the resistivities of the capacitor have increased and, therefore, electrolyte solutions with lower resistivity and higher conductivity are required to be use in the electrolytic capacitor.
A conventional electrolytic capacitor electrolyte solution generally comprises a solution of a carboxylic acid, such as adipic acid or benzoic acid, or its ammonium salt as the electrolyte dissolved in a solvent composed of ethylene glycol (EG) as the main solvent with water added to about 15 wt %. Such electrolyte solutions have resistivities of about 1.5Ω·m (150 Ω·cm).
On the other hand, the capacitor is required to have a low impedance (Z) for the purpose of achieving sufficient performance. The impedance is determined by several factors and, for example, it is lowered by increasing the electrode area of the capacitor and, therefore, large-sized capacitors inherently provide lower impedance. Improving the separator is another approach to lowering the impedance. However, the resistivity of the electrolyte solution is the major factor governing the impedance, especially in smaller capacitors.
Recently, low-resistivity electrolyte solutions have been developed using aprotic organic solvents such as GBL (γ-butyrolactone). (See, for example, Japanese Unexamined Patent Publication SHO No. 62-145713, Japanese Unexamined Patent Publication SHO No. 62-145714 and Japanese Unexamined Patent Publication SHO No. 62-145715). However, capacitors using such aprotic electrolyte solutions have vastly inferior impedances compared to solid capacitors employing electron conductors with resistivity of 1.0Ω·cm or below.
It is currently the situation that aluminum electrolytic capacitors, which employ electrolyte solutions, have poor low temperature stability and a rather large −40° C. impedance/20° C. impedance ratio Z(−40° C.)/Z(20° C.) of about 40 at 100 kHz. In light of this situation, it has been desirable to provide an aluminum electrolytic capacitor with low impedance, low resistivity and excellent low temperature stability.
Moreover, the water used as part of the solvent in an aluminum electrolytic capacitor electrolyte solution is a chemically active substance with respect to the aluminum of the anode foil or cathode foil. It therefore reacts with the anode foil or cathode foil to generate hydrogen gas, and this raises the pressure in the capacitor and increases the stress on the capacitor element, thereby causing deformation or breakage of the wound structure; this may promote fly out of the electrolyte solution to the exterior or activate the safety vent, thereby significantly altering the characteristics. In order to eliminate the problem of generated hydrogen gas, it has been attempted in the past to absorb generated hydrogen gas in an electrolytic capacitor load test or the like. For example, Japanese Examined Patent Publication SHO No. 59-15374 discloses an electrolyte solution for use in an electrolytic capacitor, which is obtained by adding a carboxylic acid and carboxylic acid ammonium salt to a solvent containing ethylene glycol added with 5-20 wt % water to prepare a buffer solution, and then further adding 0.05-3 wt % p-nitrophenol. By using this electrolyte solution it is possible to provide an electrolytic capacitor with limited boehmite production and little hydrogen gas generation on the electrode foil surface together with an enhanced low temperature stability and a long usable life.
Also, Japanese Examined Patent Publication SHO No. 63-14862 discloses an electrolyte solution for use in an electrolytic capacitor capable of exhibiting an excellent corrosion inhibiting effect for cleaning with halogenated hydrocarbons, wherein o-nitroanisole is added to an electrolyte solution prepared by dissolving an organic acid, inorganic acid or its salt as a solute in a solvent consisting mainly of ethylene glycol. It is stated in this publication that o-nitroanisole used as a corrosion inhibitor exhibits a hydrogen gas-absorbing effect, thereby absorbing the hydrogen gas generated inside the electrolytic capacitor during use and minimizing valve-opening accidents or changes in capacitance.
However, research by the present inventors has shown that although p-nitrophenol or o-nitroanisole exhibit an initial hydrogen gas absorbing effect in an electrolyte solution for use in an electrolytic capacitor with low water concentration, such as are commonly used in the prior art, they are unable to exhibit and maintain satisfactory hydrogen gas absorbing effects when the amount of water in the solvent of the electrolyte solution is 20 wt % or greater, or when the electrolytic capacitor is used for a prolonged period in a high-temperature environment.
In Japanese Unexamined Patent Publication No. 2000-173872, the present inventors have already disclosed that the aforementioned object can be achieved by an electrolyte solution for use in an electrolytic capacitor comprising nitrophenol, nitrobenzoic acid, dinitrobenzoic acid, nitroacetophenone or nitroanisole in an electrolyte solution wherein the solvent is composed of 20-80 wt % of an organic solvent and 80-20 wt % water. However, these differ from the compounds of the invention which have unsaturated bond-containing chains.
It is an object of the present invention to overcome the aforementioned drawbacks associated with the prior art, and to provide an electrolyte solution for use in an electrolytic capacitor having low impedance and excellent low temperature stability, as represented by the impedance ratio at a low temperature and an ordinary temperature, and a satisfactory usable life, and being capable of exhibiting an excellent hydrogen gas-absorbing effect even when an electrolyte solution employing a solvent mixture with a high water content is used or when the electrolytic capacitor is used in a high-temperature environment, as well as an electrolytic capacitor employing the electrolyte solution.
It is another object of the invention to provide an electrolytic capacitor wherein the capacitor element contains a solvent-soluble compound with an unsaturated bond-containing chain which can undergo a hydrogen addition reaction in an electrolyte solution composed of at least 20 wt % water in the solvent composition.